The present disclosure relates to a high-strength steel sheet, and more particularly, to a hot-dip galvanized steel sheet and a hot-dip galvannealed steel sheet, applicable to automotive body panels, and the like, due to possessing excellent formability, and methods for manufacturing the hot-dip galvanized steel sheet and the hot-dip galvannealed steel sheet.
With a focus on crashworthiness regulations and fuel efficiency in vehicles, high-strength steels are being actively used in automotive applications in order to achieve requirements for high strength, together with reductions in the weight of automotive bodies, and, in line with these trends, high-strength steels are being applied to automotive body panels. Currently, a 340 MPa grade bake hardening steel is generally used for automotive body panels. However, 490 MPa grade bake hardening steel is also being used in some applications, while the application of 590 MPa grade bake hardening steel is further expected in the future.
While the use of such bake hardening steel sheets for body panels allows for weight reductions, and improves impact resistance, the use of such steel sheets may be disadvantageous, in that formability decreases as strength increases. Thus, recently, customers have demanded steel sheets having a low yield ratio (YR=YS/TS) and excellent ductility in order to compensate for insufficient workability in high-strength steel sheets applied to automotive body panels. Furthermore, above all, steel sheets used for automotive body panels should have excellent surface qualities. However, it may be difficult to ensure desired surface qualities in plated steel sheets due to hardenability-imparting and easily oxidizable elements (e.g., Si, Mn etc.), which are added to obtain high strength.
On the other hand, excellent corrosion resistance is required for the proper use of such steel sheets in automotive applications, and hot-dip galvanized steel sheets having excellent corrosion resistance have thus been conventionally used as steel sheets for automotive applications. These steel sheets are manufactured through continuous hot-dip galvanizing equipment in which recrystallization annealing and plating are performed on the same line, and thus have an advantage in that highly corrosion resistant steel sheets may be manufactured at low cost. Furthermore, hot-dip galvannealed steel sheets, which are obtained through hot-dip galvanizing and reheating thereafter, are being widely implemented due to excellent weldability or formability in addition to excellent corrosion resistance.
Therefore, in order to reduce the weight of and increase the workability of automotive body panels, the development of a high-strength cold-rolled steel sheet having excellent formability is required, and in addition thereto, the development of a high-strength hot-dip galvanized steel sheet having excellent corrosion resistance, weldability, and formability is also required.
As prior art in which workability of such a high-strength steel sheet is improved, patent document 1 discloses a multi-phase steel sheet including martensite as a main component, and suggests a method for manufacturing a high-strength steel sheet in which fine Cu precipitates having particle diameters of 1-100 nm are dispersed in the structure to improve workability. However, according to the disclosure of patent document 1, it is necessary to add an excess amount of Cu, e.g., 2-5% of Cu, in order to precipitate fine Cu particles so that red shortness caused by Cu may occur, and manufacturing costs may be excessively increased.
Patent document 2 discloses a multi-phase steel sheet including ferrite as a primary phase, retained austenite as a secondary phase, and bainite and martensite, which are low-temperature transformed phases, and a method for improving the ductility and stretch-flangeability of the steel sheet. However, according to the disclosure of patent document 2, large amounts of Si and Al are added to obtain the retained austenite phase, and therefore it is difficult to ensure plating quality and surface qualities in steelmaking and continuous casting. In addition, there is a disadvantage in that the initial YS value is high due to transformation induced plasticity, and thus the yield ratio is high.
Patent document 3, which relates to a feature for providing a high-strength hot-dip galvanized steel sheet exhibiting good workability, discloses a steel sheet having a microstructure including a composite of relatively soft ferrite and relatively hard martensite, and a manufacturing method for improving the elongation and Lankford value (R-value) of the steel sheet. According to this feature, however, a large amount of Si is added, and thus it is difficult to obtain excellent plating quality, and manufacturing costs are increased due to the addition of large amounts of Ti and Mo.